Active object selection - separate edge
First tried to find out if it was an interior face, but it is only an edge I could delete. However I need the extra geometry for shading purposes.
Then I was pointed to Non-manifold fixes from Gleb in a Bendernation post and with the addon '3D print toolbox' I found a way to fix the geometry by cleaning up 'distorted' faces. Some extra geometry was added and the result is that the orange line isn't visible anymore.